Apparatus for electrical discharge treatment of a gas flow

ABSTRACT

An apparatus for electrical discharge treatment of a gaseous material wherein at least one gas-discharge slot reactor is electrically coupled to the secondary winding of an A.C.-supply transformer through a series combination of a switching circuit controlled by a control means and a rectifier.

llnite Stts atet- 1 Presnetsov et al.

[ APPARATUS FOR ELECTRICAL DISCHARGE TREATMENT OF A GAS FLOW [76] Inventors: Gennady Nikolaevich Presnetsov,

Ploschad Ostrovskogo, 9, kv. 25; Andrei Vladimirovich Dmitriey, Ulitsa 23, kv. 48, both of, Leningrad, USSR.

[22] Filed: June 21, 1973 [21] Appl. No.: 372,413

[52] US. Cl. 250/536, 250/531 [51] Int. Cl C0lb 13/12, BOlk 1/00 [58] Field of Search 204/164; 250/531, 535,

[56] References Cited UNITED STATES PATENTS 3.035205 5/1962 Berghaus et ul. 250/546 3,496,092 2/1970 Fraser 250/536 3,752,748 8/1973 McMillan,Jr. 250/535 Primary ExaminerT. M. Tufariello Attorney, Agent, or Firm-Waters, Roditi, Schwartz & Nissen 5 7] ABSTRACT An apparatus for electrical discharge treatment of a gaseous material wherein at least one gas-discharge slot reactor is electrically coupled .to the secondary winding of an A.C.-supply transformer through a series combination of a switching circuit controlled by a control means and a rectifier.

3 claim? 14 Drawing Figures vvvvvvvvvv PATENTED [JCT 2 2 I974 SHEET 1 BF 6 AAAAAAAAAA Dam PATENTEDUBT 22 1914 3.843882 SHEET 3 OF 6 PATENTEDnm 22 874 38431882 saw u or e I APPARATUS FOR ELECTRICAL DISCHARGE TREATMENT OF A GAS FLOW The invention relates to equipment for electrophisical treatment of gases, more particularly to an apparatus for electrical discharge treatment of a gas flow.

There exist apparatuses for electrical discharge treatment of a gas flow wherein said electrical discharge is excited by alternating current passed through a narrow slot-type discharge gap.

In the existing apparatus for electrical discharge treatment of a gas flow at least one gas-discharge slot reactor which has an inlet and an outlet for a gas flow and at least one pair of electrodes which limit the gas space and are separated by a dielectric, one of the electrodes being earthed, is electrically coupled through the second electrode to the secondary winding of a transformer, the primary winding of which is connected to an A.C. source so that the recharging of the capacitance of the dielectric barrier and of the electric capacitance of the gas space is controlled by a control means.

Besides, in the existing apparatus the A.C. source is an impulse modulator with a storage capacitor, the input of which is connected to a DC. source, and the electrical capacitance of the gas space and the capaci tance of the dielectric barrier of the gas-discharge slot reactor together with the secondary winding of the step-up transformer form an oscillatory circuit.

The main disadvantage of the existing apparatus for electrical discharge treatment of a gas flow is in that the oscillatory circuit mentioned above has at least two natural frequencies, which fact makes it impossible to excite and maintain an electrical discharge of an optimum shape in the gas space, and consequently, to control the synthesis of products in the gas material. Such oscillatory circuit develops high-frequency oscillations conductive to the formation of spark and are discharges in the gas space, which cause quick destruction of the dielectric barrier and decomposition of the end product and markedly affect the selectivity of the reactlon.

Another disadvantage of the existing apparatus is in that its A.C. source is an impulse modulator some frequencies of which cause voltage surges in the oscillatory circuit while at large power outputs the weight and dimensions of the storage capacitor incorporated in the impulse modulator are commensurate with the corresponding characteristics of the gas-discharge slot reactor and, besides, the recharging circuits of the storage capacitor cause high power losses and the use of an impulse modulator necessitates the provision of an additional D.C. source.

Still another disadvantage of the existing apparatus is the use of a step-up transformer in the oscillatory circuit, which causes additional power losses.

The existing apparatus is also disadvantageous in that each gas-discharge slot reactor requires a matched step-up transofmer, which makes the apparatus bulky not expensive.

The object of the present invention is to provide an apparatus for electrical-discharge treatment of a gas flow with an optimum shape of electrical discharge.

Another object of the invention is to provide an apparatus with a simple arrangement of the A.C. source and without an additional D.C. source.

one gas-discharge slot reactor which has an inlet and an outlet for the gas flow and at least one pair of electrodes which limit the gas space and are separated by a dielectric, one of the electrodes being earthed, iselectrically coupled by means of the second electrode to the secondary winding of a transformer, the primary winding of which is connected to an A.C. source so that the recharging of the capacitance of the dielectric barrier and the electrical capacitance of the gas space in the reactor is controlled by a control means and, according to the invention, the secondary winding of the transformer is electrically coupled to the second elec trode of the gas-discharge slot reactor by means of a series combination of a rectifier which converts alternating current to direct current and a switching circuitcontrolled by a control mains, said switching circuit alter'nately connecting during operation of the apparatus the second electrode of the gas-discharge slot reactor to the rectifier leads thereby enabling the reactor capacitances to be recharged and an electrical discharge to be excited in the gas space.

The switching circuit preferably comprises a switching element connected to the control means and a reactive element, and the first lead of the rectifier is preferably connected to the second electrode of the reactor through the switching element while the second lead of the rectifier is preferably connected to the second electrode through the reactive element, the electric characteristics of the switching and reactive elements being such as to ensure that the charge time constant of the reactor capacitances be equal to at least three times the discharge time constant of these capacitances.

A second reactive element may be connected in the switching circuit between the first lead of the rectifier and the second electrode of the reactor.

A switching element which is not controlled by the control means may be advantageously connected in series with the reactive element between the second lead of the rectifier and the second electrode of the reactor.

The switching circuit preferably comprises two switching elements connected to the control means so that each switching element should connect each lead of the rectifier with the second electrode of the reactor.

Preferably, the switching elements are connected to the second electrode of the reactor through a reactive element.

A reactive element is preferably connected in series with the switching element between each lead of the rectifier and the second electrode of the reactor.

The provision of such an apparatus makes it possible to automatically control the operating conditions of the gas-discharge slot reactor, to increase the efficiency of the entire apparatus and decrease the cost of the end product.

Other objects and advantages of the invention described herein will be better understood from the following description of its specific embodiment when read in connection with the drawings, in which:

FIG. I shows the general, partly cut away view of an apparatus or electrical discharge treatment of a gas flow, according to the invention;

FIG. 2 is a schematic diagram of an apparatus for electrical discharge treatment of a gas flow, according to the invention;

FIG. 3 is a block diagram of an apparatus for electrical discharge treatment of a gas flow with another version of the switching circuit, according to the invention;

FIG. 4 is a block diagram of an apparatus for electrical discharge treatment of a gas flow with a third ver sion of the switching circuit, according to the invention;

FIG. 5 is a block diagram showing an apparatus for electrical discharge treatment of a gas flow with a fourth version of the switching circuit, according to the invention;

FIG. 6 is a block diagram of an apparatus for electrical discharge treatment of a gas flow with a fifth version of the switching circuit, according to the invention;

FIG. 7 is a block diagram of an apparatus for electrical discharge treatment of a gas flow with a sixth version of the switching circuit, according to the invention;

FIG. 8 is a block diagram of an apparatus for electrical discharge treatment of a gas flow with a seventh version of the switching circuit, according to the invention;

FIG. 9 is a block diagram of an apparatus for electrical discharge treatment of a gas flow with an eighth version of the switching circuit, according to the invention;

FIG. 10 is a block diagram of an apparatus for electrical discharge treatment of a gas flow with a ninth version of the switching circuit, according to the invention;

FIG. 11 is a block diagram of an apparatus for electrical discharge treatment of a gas flow with a tenth version of the switching circuit, according to the invention;

FIG. 12 is a block diagram of an apparatus for electrical discharge treatment of a gas flow with an eleventh version of the switching circuit, according to the invention;

FIG. 13 is a block diagram of an apparatus for electrical discharge treatment of a gas flow with a twelfth version of the switching circuit, according to the invention;

FIGS. 14 a, b, c, and d) show the waveforms, according to the invention, of the:

control pulses (at a); the recharging current of the capacitances of one gas-discharge slot reactor (at b);

the recharging current of the capacitances of another reactor (at c);

the recharging current of the capacitances of a third reactor (at d).

Now discuss the apparatus for electrical discharge treatment of a gas flow.

An input lead 3 from the primary winding of a transformer 4 which is a conventional step-up transformer is connected to an oscillator 1 (FIG. 1) which is used as an A.C. source through a switch 2 in the form of a distribution cabinet with meters mounted on its front panel. The secondary winding of the transformer 4 is connected to a rectifier 5 which is mounted in the same unit 6 as the transformer. The output of the rectifier 5 is connected to a control board 9 through a first lead 7 and a second earthed lead 8.

An output lead 10 of the board 9 is connected by means of a cable to a high-voltage input lead 11 of each of three gas-discharge slot reactors 12, 12' and 12" which have an inlet 13 and an outlet 14 for a gas flow and at least one pair of electrodes which limit the gas space, one of the electrodes being earthed through a lead 15.

FIG. 2 shows the schematic diagram of the apparatus described herein.

The rectifier 5 is a conventional three-phase fullwave rectifier using semiconductor rectifying elements 16, a choke l7 and a capacitor-18. The leads 7 and 8 of the rectifier 5 are connected to the input of switching circuits 19, 19' and 19" associated with reactors 12, 12 and 12", respectively and consisting of two switching elements 20 and 21 in the form of thyratrons connected to a control means 24, and a reactive element 22 in the form of a choke.

Each of the reactors 12, 12' and 12" is designed as a heat exchanger with several pairs of electrodes. In the reactor 12 which is used for the production of ozone from atmospheric oxygen the number of the electrode pairs is 242. Structurally (not shown) the reactor 12 is a metal cylinder with two hundred and forty two metal tubes functioning as earthed electrodes which are mounted on the inner surface of the cylinder by means of metal grids. Each tube houses a second cylindrical electrode made from dielectric a heat-resistant glass tube and positioned coaxially by a special centering device the inner surface of the tube being coated with a conducting material electrically connected to the highvoltage input lead 11.

The inner surface of each earthed electrode and the outer surface of the heat-resistant glass tube limit the gas space.

In this case each of the switching elements 20 and 21 connects an electrode 23 of the reactor 12 with the leads 7 and 8 of the rectifier 5 through a reactive element 22 and is in its turn connected to a control means 24. The control means 24 comprises a master oscillator 25 using a transistor 26 connected in a conventional circuit, a pulse transformer 27, capacitors 28, 29 and 30 and resistors 31, 32 and 33. The output of the oscillator 25 is connected to a scaling circuit 34 which consists of six identical channels, each comprising a dynistor 35, a diode 36, capacitors 37 and 38 and a resistor 39 connected in a conventional circuit.

Each channel of the scaling circuit 34 includes an amplifier 40 which is connected to the channel through a resistor 41 and is made up of a thyristor 42, a capacitor 43 a pulse transformer 44 and resistors 45 and 46 also connected in a conventional circuit. The outputs of the amplifiers 40 are so connected to electrodes 47 and 48 of the switching elements that the output of the amplifier 40 of the first channel is connected to the switching element 20 of the switching circuit 19; the output of the amplifier 40 of the second channel is connected to the switching element 21 of the switching circuit 19"; the output of the amplifier 40 of the third channel is connected to the switching element 20 of the switching circuit 19'; the output of the amplifier 40 of the fourth channel is connected to the switching element 21 of the switching circuit 19; the output of the amplifier 40 of the fifth channel is connected to the switching element 20 of the switching circuit 19"; the output of the amplifier 40 of the sixth channel is connected to the switching element 21 of the switching circuit 19'. The control means 24 also comprises a power supply unit 49 formed by a rectifier the input of which is coupled through a switch 50 and a transformer 51 to the AC. mains and the output of which is connected to bus-bars 52 and 53.

In another version shown in FIG. 3 each switching circuit 19 differs from the switching circuit 19 shown in FIG. 2 in that it consists of one switching element 21 connected to the control means 24 and in that the reactive element 22 is used instead of the switching element 20.

A third version shown in FIG. 4 differs from the version shown in FIG. 3 in that each switching circuit 19 has one more reactive element 54 connected in series with the switching element 2 (FIG. 4).

FIG. 5 shows a fourth version off the switching circuit 19 which differs from the version shown in FIG. 3 in that a switching element 55 not controlled by the control means 24 is connected in series with the reactive element 22.

Another version of the switching circuit 19 is possible I FIG. 6) which differs from the version in FIG. 4 in that the switching element 55 I FIG. 6) not controlled by the control means 24 is connected in series with the reactive element 22.

One more possible version of the switching circuit 19 is shown in FIG. 7. This version differs from the switching circuit 19 shown in FIG. 2 in that the switching element 20 (FIG. 7) and the switching element 21 are connected to the electrode 23 of the reactor 12 directly without the use of the reactive element 22 (FIG. 2).

FIG. 8 shows another version of the switching circuit 19 shown in FIG. 6. In this version the reactive elements 22 and 56 are connected in series with each of the switching elements 20 and 21, respectively.

All switching circuits described above may be modified by using reactive elements controlled by the control means 24. Such version of the switching circuit 19 is shown in FIG. 9. In this version the element 22 is connected to one of the channels of the control means 24.

Another version of the switching circuit 19 of FIG. 8 is shown in FIG. 10. This version includes the nonlinear reactive elements 22" and 56.

FIG. 11 shows the switching circuit 19 wherein each of the switching elements 20 and 21 is switched by means of such circuit components as a choke 57, a capacitor 58, a non-adjustable switching element 59 and a resistor 60 connected as shown in FIG. 11.

The reactive element 22' (FIG. 12) may be designed in the form of two series-connected chokes 61 and 62 and a capacitor 63 connected in parallel with the choke 62.

And, finally. in the next version of the switching circuit 19 shown in FIG. 13 the reactive element is a forming line made up of the series-connected chokes 61, 62, 64 and 65 with the associated capacitors 63, 66 and 67, a switching element 68 being connected in series, and a switching element 69, in parallel, with the chokes.

Operation of an apparatus for electrical discharge treatment of a gas flow will now be described by way of example in connection with the treatment of air for the production ozone from atmospheric-oxygen.

The air stream is passed through the gas space of each of the reactors 12, 12 and 12" (FIG. 2) and simultaneously the primary winding of the transformer 4 is connected to the oscillator 1 by means of the switch 2. The rectified voltage furnished by the rectifier 5 is then applied to the switching circuits 19, 19 and 19". At the same time the power source of the control means 24 is turned on by means of the switch 50, the

master oscillator 25 is excited and the first control pulse is applied to the electrodes 47 and 48 of the switching element 20 of the switching circuits 19 1 where C, the capacitance of the dielectric barrier;

C the capacitance of the gas space.

In this case C C As a result, while the total capacitance of the reactor 12 is being charged, the potential difference across the gas space considerably exceeds that across the dielectric barrier. When the potential difference across the gas space reaches the value at which the electric breakdown occurs, an electrical discharge is excited in the space causing ozone to be produced from the oxygen contained in the passing air stream. After the electrical discharge has been struck in the gas space, the capacitance of the dielectric barrier begins to charge and after it has been fully charged, the electrical discharge extinguishes and the switching element 20 turns off.

The next control pulse is applied from the-masteroscillator 25 through the corresponding channel of the scaling circuit 34 and the amplifier 40 to the electrodes 47 and 48 of the switching element 21 of the switching circuit 19; the repetition frequency f of the pulses furnished by the master oscillator 25 is determined by the duration 1r of the charging current of the reactor 12, i.e.:

off. The next control pulse comes to the switching element 20 of the switchingcircuit 19 and the process is thus repeated.

As the characteristics of the circuits which charge and discharge the capacitances of the reactor 12 remain fixed the duration of the charging current 11 is equal to the duration of the discharge current. Therefore the recharging period T of the capacitances will be:

and the recharging frequency F of these capacitances will be:

Each of the gas-discharge slot reactors 12, 12, 12" is caused to be in turn connected to each of the leads 7 and 8 by the control pulses which successively come from the first, third and fifth channels of the scaling circuit 34 to the switching elements of the switching circuits l9, l9 and 19" and from the fourth, sixth and second channels to the switching elements 21 of the same circuits.

To ensure a high power factor of the apparatus for electrical discharge treatment of a gaseous material the duration 11' of the charging current of the reactors 12, 12' and 12", the number N of the gas-discharge slot reactors 12, 12' and 12" connected in parallel to the rectifier 5 and the recharging frequency F of each of the reactors 12, 12 and 12" must be related as follows:

In the limiting case when the recharging period T is equal to the repetition frequencyfof the pulses furnished by the master oscillator must be made equal to:

By varying the pulse repetition frequency F it is possible to adjust the efficiency of the apparatus for electrical discharge treatment of a gas flow.

When the repetition frequency f of the control pulses is varied within the limits determined by the relationship f s N 11'; the recharging frequency of the reactors l2, l2 and 12" also varies which changes the duration of the discharge in the gas space and, consequently, the time during which the flow is treated with electrical discharge. If the recharging frequency F of the reactors 12, 12' and 12 is increased, the duration of the discharge in the gas space becomes longer as a result of which the electrical power generated within the gas space and the efficiency of the gas-discharge slot reactors l2, l2 and 12" will increase. In this way the amount of power generated within the gas space and the apparatus efficiency can be adjusted within wide limits without changing the level of the voltage.

The repetition frequency of the control pulses furnished by the master oscillator 25 is changed by means of the resistor 33 which therefore can be used to adjust the efficiency of the apparatus and the concentration of the end product.

The switching circuit 19, 19' and 19" shown in FIG. 3 operate as follows. The output voltage of the rectifier 5 is applied to the reactors l2, l2 and 12" (FIG. 2) through the lead 7 and the reactive element 22 (FIG. 3) causing the capacitances of the reactors 12, 12' and 12" (FIG. 2) to be charged as was described above. Upon the arrival of a control pulse the capacitances of the reactors are discharged through the switching element 21 (FIG. 3) in the above sequence and each of the reactors l2, l2 and 12" (FIG. 2) is in turn connected to each of the leads 7 and 8 of the rectifier 5. While the capacitances of the reactor 12 (FIG. 3) are discharging through the switching element 21, the two leads 7 and 8 of the rectifier remain connected to the reactive element 22. Therefore to ensure switching of the element 21 the reactance of the reactive element 22 is selected (considering the connecting cables) such that the charge time constant of the capacitances of the reactor 12 is at least three times as long as the discharge time constant of these capacitances.

The switching conditions are improved and the limits within which the switching element 21 (FIG. 4) operates reliably are increased if the reactive element 54 is connected in its circuit.

To ensure the resonant character of charging of the capacitances of the reactor 12 the non-controllable switching element 55 and the reactive element 22 are connected in series enabling these capacitances to be charged to higher voltages, and prolonging the duration of the discharge in the gas space and, therefore, increase the efficiency of the reactor 12.

The two effects mentioned above are obtained through the use of a circuit shown in FIG 6, which operates similarly to the last two circuits discussed above.

The switching circuit 19 shown in FIG. 7 differs from the switching circuits shown in FIGS. 3, 4, 5 and 6 in that it has better switching conditions as the switching elements 20 and 21 of this circuit turn off after the capacitances of the reactor 12 have been recharged.

The switching circuit 19 (FIG. 8) differs from the switching circuit 19 shown in FIGS. 2 and 7 in that it enables the pulses of the charging and discharging current to be formed separately by selecting the appropriate characteristics of the reactive elements 22 and 56, which makes electrical discharge treatment of the gas flow more effective.

The controlled reactive element 22 makes it possible not only to vary the charge and discharge time of the capacitances of the reactor 12 but also-to adjust the dencity of the charging current within the gas space. As a result the processes which take place in the reactor 12 can be adjusted with respect to any characteristic efficiency, concentration, selectivity).

A constant density of the charging and discharging current within the gas space of the reactor 12 is maintained by the use of the non-linear reactive elements 22" and 56 in the switching circuit 19 (FIG. 10), which maintain the power generated within the gas space at a constant, level.

At the beginning and at the end of the time 11 during which the charging (discharging) current flows, there is no electrical discharge within the gas space of the reactor 12 and the duration of such currents is equal to 30 40 percent of the capacitance recharging time T of the reactor 12. This undesirable effect is eliminated in the switching circuit 19 shown in FIGS. ll, 12 and 13.

In fact the ratings of the capacitor 58 (FIG. 11) and the choke 57 are so selected that when the charging (discharging) current reaches a threshold value at which the capacitances of the reactor 12 are still dis- A smaller capacitance recharging time T of the reactor 12 is obtained through the use of the switching circuit 19 (FIG. 12) wherein the reactor capacitances are charged in an accelerated manner through the choke 61 and the capacitor 63.

Apart from the accelerated charging the reactor capacitances at the beginning of the charging discharging) current flow, the switching circuit 19 (FIG. 13) makes it possible to obtain a nearly linear rise of the voltage at the electrodes of the reactor 12 and thus to ensure a constant current density within the gas space.

The use of the last three switching circuits 19 (FIGS. ll, 12 and 13) results in a higher recharging frequency of the reactor 12 and, consequently, a higher power and efficiency of the entire apparatus.

The apparatus for electrical discharge treatment of a gas flow may be employed at ozonation stations for purifying pot and waste water, for obtaining hydrozine and oxides of halogens and hydrocarbons and for cleaning exhaust gases by an azone-catalytic method.

The use of the apparatus for electrical discharge treatment of a gas flow makes it possible to increase the efficiency of a gas-discharge slot reactor 1.5 to 3 times by prolonging the duration of electrical discharge without making any changes in the design of the reactor.

Moreover, by selecting the ratings of the reactive elements an optimum waveform of the current in the gas space is ensured and the possibility of sparking is reduced, which limits decomposition of ozone (or any other gas produced by the apparatus) in the discharge channel, raises the efficiency of the reactor 12 and cuts down power consumption per unit of end product.

In the apparatus described herein each gas-discharge slot reactor 12 consumes 6.8 to 12 kW-hrs for production of one kilogram of ozone from an ozono-air mixture with ozone concentration of l8 l9 gr/m The use of the apparatus makes it possible to increase the efficiency of the reactor 1.5 to 2 times, make the treatment of a gas flow automatic and ensure the required operating conditions of the reactors 12, 12 and 12" (FIG. 2) both by adjusting the rate of change of the voltage at the reactors through the adjustment of the reactive element 22' (FIG. 8) and by changing the capacitance recharging frequency F with the help of the control means 24 (FIG. 2).

Another advantage of the apparatus is in that the operating conditions of the reactors l2, l2 and 12" can be adjusted without the use of special frequency converters, and the overall dimensions and weight of the apparatus are reduced accordingly.

Further reduction in the dimensions of the apparatus is obtained due to the fact that transformers do not have to be provided for each reactor.

Finally, provisions are made in the apparatus for any number of the gas-discharge slot reactors 12 to be connected both in series and in parallel to the rectifier 5.

What we claim is:

1. An apparatus for electrical discharge treatment of a gas flow comprising: at least one gas-discharge slot reactor which has an inlet and an outlet for a gas flow; at least one pair of electrodes for each gas-discharge slot reactor limiting the gas space thereof; a first electrode of said gas-discharge slot reactor connected to earth; a dielectric disposed between each pair of said electrodes; a second electrode of said gas-discharge re actor separated by said dielectric from said first electrode; a switching circuit for each said reactor, connected with said electrodes of said reactor; a rectifier with an input lead and an output lead converting alternating current to direct current and connected to said switching circuit; a control means connected to said switching circuit and alternately switching during operation of said apparatus said second electrode of said reactor to the leads of said rectifier so as to recharge the capacitances of said dielectric and said gas space of said reactor and excite an electrical discharge within said gas space; a transformer the secondary winding of which is connected to the input of said rectifier; and AC. source the output of which is connected to the primary winding of said transformer.

2. An apparatus as claimed in claim 1 wherein said switching circuit comprises: a switching element which is connected to said control means and couples the first lead of said rectifier with said second electrode of said reactor; a reactive element which connects the second lead of said rectifier with said second electrode of said reactor, the electrical characteristics of all of said elements being selected such that the charge time constant of said reactor capacitances is at least three times as long as the discharge time constant of said capacitances.

3. An apparatus as claimed in claim 2 wherein a switching circuit comprises a second reactive element connected between said first lead of said rectifier and said second electrode of said reactor in series with said switching element.

4. An apparatus as claimed in claim 2 wherein a switching element is connected between said second lead of said rectifier and said second electrode of said reactor in series with said reactive element.

5. An apparatus as claimed in claim 3 wherein a switching element is connected between said second lead of said rectifier and said second electrode of said reactor in series with said first reactive element.

6. An apparatus as claimed in claim l wherein theswitching circuit comprises a first switching element which is connected to said control means and couples said first lead of said rectifier to said second electrode of said reactor; a second switching element which is connected to said control means and couples said second lead of said rectifier to said second electrode of said reactor.

7. An apparatus as claimed in claim 6 wherein said switching elements are coupled to said second electrode of said reactor through a reactive element.

8. An apparatus as claimed in claim 6 wherein a reactive element is connected between each of said leads of said rectifier and said second electrode of said reactor in series with said switching element. 

1. An apparatus for electrical discharge treatment of a gas flow comprising: at least one gas-discharge slot reactor which has an inlet and an outlet for a gas flow; at least one pair of electrodes for each gas-discharge slot reactor limiting the gas space thereof; a first electrode of said gas-discharge slot reactor connected to earth; a dielectric disposed between each pair of said electrodes; a second electrode of said gas-discharge reactor separated by said dielectric from said first electrode; a switching circuit for each said reactor, connected with said electrodes of said reactor; a rectifier with an input lead and an output lead converting alternating current to direct current and connected to said switching circuit; a control means connected to said switching circuit and alternately switching during operation of said apparatus said second electrode of said reactor to the leads of said rectifier so as to recharge the capacitances of said dielectric and said gas space of said reactor and excite an electrical discharge within said gas space; a transformer the secondary winding of which is connected to the input of said rectifier; and A.C. source the output of which is connected to the primary winding of said transformer.
 2. An apparatus as claimed in claim 1 Wherein said switching circuit comprises: a switching element which is connected to said control means and couples the first lead of said rectifier with said second electrode of said reactor; a reactive element which connects the second lead of said rectifier with said second electrode of said reactor, the electrical characteristics of all of said elements being selected such that the charge time constant of said reactor capacitances is at least three times as long as the discharge time constant of said capacitances.
 3. An apparatus as claimed in claim 2 wherein a switching circuit comprises a second reactive element connected between said first lead of said rectifier and said second electrode of said reactor in series with said switching element.
 4. An apparatus as claimed in claim 2 wherein a switching element is connected between said second lead of said rectifier and said second electrode of said reactor in series with said reactive element.
 5. An apparatus as claimed in claim 3 wherein a switching element is connected between said second lead of said rectifier and said second electrode of said reactor in series with said first reactive element.
 6. An apparatus as claimed in claim 1 wherein the switching circuit comprises a first switching element which is connected to said control means and couples said first lead of said rectifier to said second electrode of said reactor; a second switching element which is connected to said control means and couples said second lead of said rectifier to said second electrode of said reactor.
 7. An apparatus as claimed in claim 6 wherein said switching elements are coupled to said second electrode of said reactor through a reactive element.
 8. An apparatus as claimed in claim 6 wherein a reactive element is connected between each of said leads of said rectifier and said second electrode of said reactor in series with said switching element. 